In electrical and electronic engineering, a device to measure current through a conductor is a clamp or current-probe with two jaws, which open and clamp around a wire. This allows the electrical current in the conductor to be measured without having to make physical contact with it, or having to disconnect it for insertion through the probe. An electrical meter with an integral current clamp is known as a clamp meter, clamp-on ammeter, or tong tester. Several types exist:                1. Rectifiers: Less-expensive clamp meters use a rectifier circuit which actually reads mean current, but are calibrated to display the RMS current corresponding to the measured mean, only giving a correct RMS reading if the current is a sine wave. The readings produced by such meters can be quite inaccurate. Meters, which respond to true RMS rather than mean-current, are called “true RMS” meters.        2. Split Ferrite Ring: A common form of clamp meter uses a split ferrite ring. A wire coil is wound around one or both halves, forming the secondary winding of a current transformer, where the conductor to be measured acts like the primary winding. Like any transformer this type only works with AC or pulse waveforms, with some instruments extending into the megahertz range.        3. Iron Vane: In the iron vane type, the magnetic flux in the conductor directly affects a moving iron vane, allowing both AC and DC to be measured and gives a true RMS value for non-sinusoidal AC waveforms. Due to its physical size it is generally limited to power transmission frequencies up to around 100 Hz. The vane is usually fixed directly to the display mechanism of an analogue (moving pointer) clamp meter. The iron jaws of the meter direct the magnetic field surrounding the conductor to an iron vane that is attached to the needle of the meter. The iron vane moves in proportion to the strength of the magnetic field and thus produces a meter indication proportional to the current.        4. Hall-Effect: The Hall-Effect type is more sensitive and is able to measure both DC and AC; some instruments measure up to the kilohertz range. This type is often used with oscilloscopes and with high-end computerized digital multi-meters. Typical handheld Hall Effect units can read currents of 200 mA and even lower.        5. Open Jaw: Hall Effect probes can also be of the “open jaw” design. Their advantage is the ease to measure AC current without having to open the jaws, making this mechanical part unnecessary. Test-leads are often added to use this instrument as a regular multi-function meter.        
The above known meters have certain shortcomings. Conventional current-clamp meters can only measure current in a single-conductor cable, requiring the conductors of multi conductor cables to be separated before the current in one conductor can be measured. It is against the electrical installation code of residential and industrial wiring to open cables or to separate wires. If more than one conductor were to be passed through, the measurement would be the vector sum of the currents flowing in the conductors. If the clamp is closed around a two-conductor cable in which the same current flows down one conductor and up the other, the conventional meter will provide a reading of zero.